In semiconductor processing of wafers using ion implantation, it is customary to "dope" the wafers using an ion beam consisting of a selected species of ion to impart desired electrical properties to the semiconductor wafer. Common ions used for this purpose are boron, arsenic, phosphorus and antimony.
In the prior art, two methods for producing beams of boron, arsenic and phosphorus are known. In one method, a poisonous or highly toxic feed gas is supplied from a pressurized gas supply to an ion source, where ions are created by plasma induced breakdown of the feed gas. Ions of the desired species are then extracted from the source and accelerated toward the target wafer using well known ion implantation apparatus. For example, boron ions are typically produced by flowing boron trifluoride (BF.sub.3) into the source where ions are produced by plasma induced breakdown of the boron trifluoride. Arsenic atoms are typically produced using arsine (AsH.sub.3) as the feed gas. Similarly, phosphorous ions are typically produced using phosphine (PH.sub.3) as the feed gas. The gases supplied to the source are housed in a gas box containing three or four gas cylinders or bottles. In a typical three-gas manifold, source gases BF.sub.3, AsH.sub.3 and PH.sub.3 are each contained in a separate gas cylinder in the gas box. Each cylinder is typically pressurized up to approximately 400 psi. Table 1 (from Section 1.2, Gas Bottle Charge Procedures Serial Process Ion Implantation Systems, Varian/Extrion Division, Glouster, Mass., Feb. 18, 1985) describes the physical characteristics and hazards of these commonly used implanter gases.
TABLE 1 ______________________________________ PHYSICAL CHARACTERISTICS AND HAZARDS OF IMPLANTER GASES Smell Gas Characteristics Hazards ______________________________________ Boron Pungent Colorless, heavier Toxic when in- Trifluoride and than air. Generates haled. Severely (BF.sub.3) Suffocating white fumes when corrosive to skin, exposed to air. eyes and mucous Sharp, irritating membranes. Skin odor in low concen- contact with va- tration. por liquid can cause serious burns. Phosphine Decaying Colorless, heavier Extremely (PH.sub.3) Fish than air. Gaseous toxic, highly flam- at room tempera- mable. Kidney, ture and atmos- heart and brain pheric pressure. appear particular- Minimum warning ly sensitive to concentration 1.4 PH.sub.3 damage. to 2.8 ppm (parts per million). Leave area immediately until clear. Arsine Garlic- Colorless, heavier Extremely toxic, (AsH.sub.3) like than air. Gaseous highly flammable. at room tempera- A nerve and ture and atmos- blood poison. pheric pressure. Difficult to detect by smell, despite distinctive odor. Leave area imme- diately if AsH.sub.3 is suspected. ______________________________________
Boron trifluoride is designated as a hazardous material by the Department of Transportation (DOT) in the hazard class of non-flammable gas requiring the label non-flammable gas and poison (see 49 CFR .sctn.172.101). The Occupational Safety and Health Administration (OSHA) and the American Conference of Governmental Industrial Hygienists (ACGIH) have established a ceiling limit of 1 ppm for this gas. Arsine is designated by DOT as a hazardous material in the hazard class of Poison A requiring the labels poison gas and flammable gas (see 49 CFR .sctn.172.101). DOT defines a class A poison as a poisonous gas or liquid of such nature that a very small amount of the gas or vapor of the liquid, mixed with air is dangerous to life (see 49 CFR .sctn.173.326(a)). Both OSHA and ACGIH impose an exposure limit of 0.05 ppm for arsine. Arsine is highly toxic and may be fatal if inhaled. Phosphine is also classified by DOT as a hazardous material in the hazard class of Poison A requiring the labels poison gas and flammable gas (see 49 CFR, .sctn. 172.101).
The handling of these hazardous and poisonous gases requires training and special safety precautions and special clothing and safety apparatus including self-contained breathing apparatus for those personnel who change or replace gas cylinders in the gas box of an ion implanter from time to time. For example, it is typically required for personnel transporting or handling BF.sub.3 cylinders to wear a full-face shield or chemical safety glasses together with protective clothing that covers the entire body including chemically resistant gloves and clothed shoes. Personnel are also required to wear fire resistant garments when working with arsine and phosphine gases.
A second prior art method for producing boron, arsenic or phosphorous ions in ion implanters comprises simple thermal vaporization of these respective solid elements or a thermally induced chemical reaction such as the decomposition of lithium fluoroborate (LiBF.sub.4) to liberate boron trifluoride. Solid arsenic is classified as a poison B by the DOT in 49 CFR, .sctn.172.101 and dry phosphorus is classified as a hazardous material in the hazard class of flammable solid requiring a label of a flammable solid and poison. The use of such solids reduces the amount of volatile poisonous vapors present in the system, but these oven type sources are troublesome to use. Typically, the plasma chamber and feed tubes must operate at high temperature to avoid plugging, typically at temperatures exceeding 700.degree. C. This leads to enhanced corrosion. The control of temperature is the principal means for controlling the rate of feed of the arsenic, boron or phosphorus vapors and this feed mechanism has a long response time due to the thermal inertia of the oven, which leads to difficulties in controlling the vapor pressure.
Ion beams have also been produced by sputtering solid materials within the source. The use of physical sputtering to vaporize low pressure solids without chemical enhancement leads only to very low ion currents (less than 1 milliamp, see K. J. Hill and R. S. Nelson, Nuclear Instruments and Methods, Vol. 38, p. 15 (1965)) and so this type of source is not used in commercial semiconductor implanters. The work of Hill and Nelson has been confirmed by the applicants in the case of boron.